Method for recycling denitration catalyst

ABSTRACT

There is provided a method for recycling a catalyst that exhibits a high denitration efficiency at a relatively low temperature and does not cause oxidation of SO 2  in a selective catalytic reduction reaction that uses ammonia as a reducing agent. A method for recycling a denitration catalyst includes a step of removing a used denitration catalyst from a denitration device and then coating the used denitration catalyst with a catalyst component. The catalyst component contains 43 wt % or more of vanadium pentoxide and has a BET specific surface area of 30 m 2 /g or more, and the denitration catalyst after recycling is used for denitration at 200° C. or lower.

TECHNICAL FIELD

The present invention relates to a method for recycling a denitrationcatalyst. More specifically, the present invention relates to a methodfor recycling a denitration catalyst used when exhaust gas generatedthrough combustion of fuel is cleaned up.

BACKGROUND ART

One of pollutants emitted to the air through fuel combustion is nitrogenoxide (NO, NO₂, NO₃, N₂O, N₂O₃, N₂O₄, or N₂O₅). Nitrogen oxide causes,for example, acid rain, ozone depletion, and photochemical smog andseriously affects the environment and the human body, and therefore thetreatment for nitrogen oxide has been an important issue.

A known technique of removing the nitrogen oxide is a selectivecatalytic reduction reaction (NH₃-SCR) that uses ammonia (NH₃) as areducing agent. As described in Patent Document 1, a catalyst in whichvanadium oxide is supported on titanium oxide serving as a carrier iswidely used as a catalyst for the selective catalytic reductionreaction. Titanium oxide is the best carrier because titanium oxide hasa low activity against sulfur oxide and has high stability.

Patent Document 1: Japanese Unexamined Patent Application, PublicationNo. 2004-275852

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

On the other hand, vanadium oxide plays a key role in the NH₃-SCR, butvanadium oxide cannot be supported in an amount of about 1 wt % or morebecause vanadium oxide oxidizes SO₂ into SO₃. Therefore, vanadium oxideis typically used in an amount of 1 wt % or less relative to itscarrier. Furthermore, in the current NH₃-SCR, a catalyst in whichvanadium oxide (and tungsten oxide in some cases) is supported on atitanium oxide carrier hardly reacts at low temperature and thus needsto be used at a high temperature of 350° C. to 400° C.

In order to increase the degree of freedom in the design of apparatusesand facilities with which the NH₃-SCR is performed and increase theefficiency, the development of a catalyst having a high nitrogen oxidereduction activity even at low temperature has been demanded. With thisdevelopment, a method for recycling such a catalyst has also beendemanded.

In view of the foregoing, it is an object of the present invention toprovide a catalyst that exhibits a high denitration efficiency at lowtemperature and does not cause oxidation of SO₂ in a selective catalyticreduction reaction that uses ammonia as a reducing agent.

Means for Solving the Problems

The present invention relates to a method for recycling a denitrationcatalyst, the method including a step of removing a used denitrationcatalyst from a denitration device and then coating the used denitrationcatalyst with a catalyst component, wherein the catalyst componentcontains 43 wt % or more of vanadium pentoxide and has a BET specificsurface area of 30 m²/g or more, and the denitration catalyst afterrecycling is used for denitration at 200° C. or lower.

The used denitration catalyst is preferably coated with the catalystcomponent by impregnating the used denitration catalyst with a chelatecompound solution in which vanadium pentoxide is dissolved and thenperforming firing.

The used denitration catalyst is preferably coated with the catalystcomponent by spraying a chelate compound solution in which vanadiumpentoxide is dissolved onto the used denitration catalyst and thenperforming firing.

In the catalyst component, an amount of NH₃ desorbed by NH₃-TPD (TPD:temperature programmed desorption) is preferably 10.0 μmol/g or more.

Effects of the Invention

The denitration catalyst that has been recycled by the recycling methodaccording to the present invention exhibits a high denitrationefficiency particularly at 200° C. or lower, which allows detoxificationof NO into N₂. The selective catalytic reduction reaction that uses thedenitration catalyst which has been recycled by the recycling methodaccording to the present invention can be performed at 200° C. or lower,which does not cause oxidation of SO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the powder X-ray diffraction results of vanadiumpentoxide catalysts produced in Example 1, Reference Examples 1 and 2,and Comparative Example 1.

FIG. 2 illustrates the powder X-ray diffraction results of vanadiumpentoxide catalysts produced in Examples 1 and 2, Reference Examples 3to 6, and Comparative Examples 2 and 3.

FIG. 3 illustrates the NH₃-SCR activity of vanadium pentoxide catalystsproduced in Example 1, Reference Examples 1 and 2, and ComparativeExamples 1 and 4.

FIG. 4 illustrates the relationship between the reaction temperature andthe N₂ selectivity in a selective catalytic reduction reaction that usesvanadium pentoxide catalysts produced in Reference Example 1 andComparative Example 1.

FIG. 5 illustrates the space velocity dependency in the case where avanadium pentoxide catalyst produced in Reference Example 1 is used inan NH₃-SCR reaction.

FIG. 6 illustrates a change in the NO conversion ratio over time in thecase where a vanadium pentoxide catalyst produced in Reference Example 1is used in a selective catalytic reduction reaction in coexistence withwater.

FIG. 7 illustrates changes in the NH₃, NO, and SO₂ concentrations overtime in the case where a vanadium pentoxide catalyst produced inReference Example 1 is used in a selective catalytic reduction reactionin coexistence with S.

FIG. 8 illustrates the relationship between the amount of vanadiumpentoxide supported and the NO conversion ratio of a vanadium pentoxidecatalyst produced in each of Examples at each reaction temperature.

FIG. 9 illustrates the relationship between the BET specific surfacearea and the NO conversion ratio of a vanadium pentoxide catalystproduced in each of Examples, Reference Examples, and ComparativeExamples.

FIG. 10 illustrates the powder X-ray diffraction results of vanadiumpentoxide catalysts produced in Examples 4 to 6 and Reference Examples 7and 8.

FIG. 11 illustrates the NH₃-SCR activity of vanadium pentoxide catalystsproduced in Examples 4 to 6 and Reference Examples 7 and 8.

FIG. 12 illustrates the relationship between the specific surface areaand the NO conversion ratio of vanadium pentoxide catalysts produced inExamples 4 to 6, Reference Examples 1, 2, and 7, and Comparative Example1.

FIG. 13 illustrates the relationship between the BET specific surfacearea and the amount of NH₃ desorbed of vanadium pentoxide catalystsproduced in Examples 4 and 5, Reference Examples 1 and 2, andComparative Example 1.

FIG. 14 illustrates the relationship between the amount of NH₃ desorbedand the NO conversion ratio of vanadium pentoxide catalysts produced inExamples 4 and 5, Reference Examples 1 and 2, and Comparative Example 1.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereafter, embodiments of the present invention will be described.

A method for recycling a denitration catalyst according to the presentinvention includes a step of removing a used denitration catalyst from adenitration device and then coating the used denitration catalyst with acatalyst component. The catalyst component contains 43 wt % or more ofvanadium pentoxide and has a BET specific surface area of 30 m²/g ormore, and the denitration catalyst after recycling is used fordenitration at 200° C. or lower. Such a denitration catalyst can exhibita high denitration effect even in a low-temperature environment comparedwith known denitration catalysts such as a vanadium/titanium catalyst.

Specifically, when a denitration catalyst containing 3.3 wt % or more ofvanadium oxide in terms of vanadium pentoxide is used in a selectivecatalytic reduction reaction (NH₃-SCR) that uses ammonia as a reducingagent, the NO conversion ratio is approximately 35% or more at areaction temperature of 120° C. and approximately 60% or more at areaction temperature of 150° C. Even at a reaction temperature of 100°C., the NO conversion ratio exceeds 20%. In contrast, if the denitrationcatalyst contains only less than 3.3 wt % of vanadium oxide in terms ofvanadium pentoxide, the NO conversion ratio is less than 20% at areaction temperature of 120° C. and even at a reaction temperature of150° C.

As described above, the catalyst component used for coating in themethod for recycling a denitration catalyst according to the presentinvention contains 43 wt % or more of vanadium oxide in terms ofvanadium pentoxide, and may also contain titanium oxide as anothercomponent in addition to the vanadium oxide. Furthermore, a noble metal,a base metal, and a main group metal may be contained. Preferably, forexample, tungsten oxide, chromium oxide, and molybdenum oxide can alsobe contained.

It has been described that the catalyst component used for coating inthe method for recycling a denitration catalyst according to the presentinvention preferably contains 43 wt % or more of vanadium oxide in termsof vanadium pentoxide. Preferably, the catalyst component may contain 80wt % or more of vanadium oxide in terms of vanadium pentoxide. Morepreferably, the content of vanadium oxide in the catalyst component maybe 100%.

The above-described vanadium oxide includes vanadium(II) oxide (VO),vanadium(III) trioxide (V₂O₃), vanadium(IV) dioxide (V₂O₄), andvanadium(V) pentoxide (V₂O₅), and the V element in vanadium pentoxide(V₂O₅) may have a pentavalent, tetravalent, trivalent, or divalent formin the denitration reaction.

Regarding the BET specific surface area of the catalyst component, forexample, in the NH₃-SCR that is performed at a reaction temperature of120° C. using a denitration catalyst containing vanadium pentoxide andhaving a BET specific surface area of 13.5 m²g⁻¹, the NO conversionratio exceeds 20%. Even in the NH₃-SCR that is performed at a reactiontemperature of 120° C. using a denitration catalyst containing vanadiumpentoxide and having a BET specific surface area of 16.6 m²g⁻¹, the NOconversion ratio exceeds 20%. In contrast, in the NH₃-SCR that isperformed at a reaction temperature of 120° C. using, for example, adenitration catalyst having a BET specific surface area of 4.68 m²/g,which is a denitration catalyst having a BET specific surface area ofless than 10 m²/g, the NO conversion ratio falls below 20%.

The BET specific surface area of the catalyst component is 10 m²/g ormore and may be preferably 15 m²/g or more. More preferably, the BETspecific surface area of the catalyst component may be 30 m²/g. Morepreferably, the BET specific surface area of the catalyst component maybe 40 m²/g or more. More preferably, the BET specific surface area ofthe catalyst component may be 50 m²/g or more. More preferably, the BETspecific surface area of the catalyst component may be 60 m²/g or more.

The BET specific surface area of the catalyst component is preferablymeasured in conformity with the conditions specified in JIS Z 8830:2013.Specifically, the BET specific surface area can be measured by a methoddescribed in Examples below.

The denitration catalyst recycled by the recycling method of the presentinvention is used for denitration at 200° C. or lower. Preferably, thedenitration catalyst is used for denitration at 160° C. or higher and200° C. or lower. Thus, oxidation of SO₂ into SO₃ does not occur duringthe NH₃-SCR reaction.

Regarding the amount of NH₃ desorbed by NH₃-TPD (TPD: temperatureprogrammed desorption), when the amount of NH₃ desorbed exceeds 10.0μmol/g, the NO conversion ratio of the denitration catalyst in theNH₃-SCR at a reaction temperature of 120° C. is 20% or more. Incontrast, when the amount of NH₃ desorbed falls below 10.0 μmol/g, theNO conversion ratio of the denitration catalyst in the NH₃-SCR at areaction temperature of 120° C. falls below 20%.

In the catalyst component used for coating in the recycling method ofthe present invention, the amount of NH₃ desorbed by NH₃-TPD (TPD:temperature programmed desorption) is 10.0 μmol/g or more. Preferably,the amount of NH₃ desorbed by NH₃-TPD may be 20.0 μmol/g or more. Morepreferably, the amount of NH₃ desorbed by NH₃-TPD may be 50.0 μmol/g ormore. More preferably, the amount of NH₃ desorbed by NH₃-TPD may be 70.0μmol/g or more.

The catalyst component of the denitration catalyst containing 43 wt % ormore of vanadium pentoxide and having a BET specific surface area of 10m²/g or more can be produced by any of a thermal decomposition process,a sol-gel process, and an impregnation process. Hereafter, a method forproducing the denitration catalyst containing 3.3 wt % or more ofvanadium pentoxide and having a specific surface area of 10 m²/g or moreby a thermal decomposition process, a sol-gel process, or animpregnation process will be described.

The thermal decomposition process includes a step of thermallydecomposing a vanadate. Examples of the vanadate that may be usedinclude ammonium vanadate, magnesium vanadate, strontium vanadate,barium vanadate, zinc vanadate, tin vanadate, and lithium vanadate.

In the thermal decomposition process, the vanadate is preferablythermally decomposed at 300° C. to 400° C.

The sol-gel process includes a step of dissolving a vanadate in achelate compound, performing drying, and performing firing. Examples ofthe chelate compound that may be used include compounds having aplurality of carboxy groups, such as oxalic acid and citric acid;compounds having a plurality of amino groups, such as acetylacetonateand ethylenediamine; and compounds having a plurality of hydroxy groups,such as ethylene glycol.

The sol-gel process preferably includes a step of dissolving a vanadatein a chelate compound such that the molar ratio of vanadium and thechelate compound is, for example, 1:1 to 1:5, though this is dependenton the chelate compound. Preferably, the molar ratio of the vanadate andthe chelate compound may be 1:2 to 1:4.

The impregnation process includes a step of dissolving a vanadate in achelate compound, adding a carrier, performing drying, and thenperforming firing. Examples of the carrier that may be used includetitanium oxide, aluminum oxide, and silica. As above, examples of thechelate compound that may be used include compounds having a pluralityof carboxy groups, such as oxalic acid and citric acid; compounds havinga plurality of amino groups, such as acetylacetonate andethylenediamine; and compounds having a plurality of hydroxy groups,such as ethylene glycol.

In the impregnation process, xwt % V₂O₅/TiO₂ (x≥43) may be produced asthe denitration component of the denitration catalyst according to anembodiment of the present invention by, for example, dissolving ammoniumvanadate in an oxalic acid solution, adding titanium oxide (TiO₂)serving as a carrier, performing drying, and then performing firing.

The thus-produced denitration component of the denitration catalystnormally contains 3.3 wt % or more of vanadium pentoxide and has aspecific surface area of 10 m²/g or more.

In the case where the surface of the denitration catalyst is unusablydeteriorated by various substances contained in the fuel after thedenitration catalyst is set in a denitration device, the useddenitration catalyst can be recycled by removing the used denitrationcatalyst from a catalyst device and coating the surface of the useddenitration catalyst with the above catalyst component.

Specifically, a used denitration catalyst removed from a denitrationdevice is immersed in a chelate compound solution in which vanadiumpentoxide is dissolved, dried, and then fired. Thus, the useddenitration catalyst can be recycled by being coated with vanadiumpentoxide. Alternatively, a used denitration catalyst removed from adenitration device is subjected to spraying with a chelate compoundsolution in which vanadium pentoxide is dissolved, dried, and thenfired. Thus, the used denitration catalyst can be recycled by beingcoated with vanadium pentoxide. As above, examples of the chelatecompound that may be used include compounds having a plurality ofcarboxy groups, such as oxalic acid and citric acid; compounds having aplurality of amino groups, such as acetylacetonate and ethylenediamine;and compounds having a plurality of hydroxy groups, such as ethyleneglycol.

The molar ratio of the vanadium pentoxide and the chelate compound ispreferably 1:1 to 1:5. Preferably, the molar ratio of the vanadiumpentoxide and the chelate compound may be 1:2 to 1:4. The firingtemperature is preferably, for example, about 300° C.

The denitration catalyst according to the above embodiment produces thefollowing effects.

(1) As described above, the method for recycling a denitration catalystaccording to the above embodiment includes a step of removing a useddenitration catalyst from a denitration device and then coating the useddenitration catalyst with a catalyst component. The catalyst componentcontains 43 wt % or more of vanadium pentoxide and has a BET specificsurface area of 10 m²/g or more. The denitration catalyst afterrecycling is used for denitration at 200° C. or lower. By using thisdenitration catalyst recycled by the recycling method according to theabove embodiment, a high denitration effect can be produced even in aselective catalytic reduction reaction at 200° C. or lower. Furthermore,in the selective catalytic reduction reaction that uses the denitrationcatalyst recycled by the recycling method according to the aboveembodiment, a high denitration effect is produced without oxidizing SO₂.

(2) As described above, the used denitration catalyst is preferablycoated with the catalyst component by impregnating the used denitrationcatalyst with a chelate compound solution in which vanadium pentoxide isdissolved and then performing firing. This increases the specificsurface area of the denitration catalyst recycled by the recyclingmethod according to the above embodiment, which improves the denitrationeffect in the selective catalytic reduction reaction that uses thedenitration catalyst according to the above embodiment.

(3) As described above, the used denitration catalyst is preferablycoated with the catalyst component by spraying a chelate compoundsolution in which vanadium pentoxide is dissolved onto the useddenitration catalyst and then performing firing. This increases thespecific surface area of the denitration catalyst recycled by therecycling method according to the above embodiment, which improves thedenitration effect in the selective catalytic reduction reaction thatuses the denitration catalyst according to the above embodiment.

(4) As described above, in the catalyst component, the amount of NH₃desorbed by NH₃-TPD (TPD: temperature programmed desorption) ispreferably 10.0 μmol/g or more. Thus, when the denitration catalystrecycled by the recycling method according to the above embodiment isused in the NH₃-SCR at a reaction temperature of 120° C., the NOconversion ratio exceeds 20%.

The present invention is not limited to the above embodiment, and any ofmodifications, improvements, and the like are included in the presentinvention as long as the object of the present invention is achieved.

EXAMPLES

Hereafter, Examples of the catalyst component of the present inventionwill be specifically described together with Reference Examples andComparative Examples. The present invention is not limited by Examples.

1. Relationship Between Vanadium Oxide Content and Specific Surface Areaand NH₃-SCR Activity 1.1 Examples and Comparative Examples ReferenceExample 1

Ammonium vanadate (NH₄VO₃) was thermally decomposed in the air at 300°C. for 4 hours to obtain vanadium pentoxide (V₂O₅). The obtainedvanadium pentoxide was used as a denitration catalyst in ReferenceExample 1. The sample name of the denitration catalyst in ReferenceExample 1 was “V₂O_(5_)300”.

Reference Example 2

Ammonium vanadate was thermally decomposed in the air at 400° C. for 4hours to obtain vanadium pentoxide. The obtained vanadium pentoxide wasused as a denitration catalyst in Reference Example 2. The sample nameof the denitration catalyst in Reference Example 2 was “V₂O_(5_)400”.

Comparative Example 1

Ammonium vanadate was thermally decomposed in the air at 500° C. for 4hours to obtain vanadium pentoxide. The obtained vanadium pentoxide wasused as a denitration catalyst in Comparative Example 1. The sample nameof the denitration catalyst in Comparative Example 1 was “V₂O_(5_)500”.

Example 1

Ammonium vanadate was dissolved in an oxalic acid solution (molar ratioof vanadium:oxalic acid=1:3). After ammonium vanadate was completelydissolved, water in the solution was evaporated on a hot stirrer, anddrying was performed in a dryer at 120° C. for one night. Then, thedried powder was fired in the air at 300° C. for 4 hours. The vanadiumpentoxide after firing was used as a denitration catalyst in Example 1.The sample name of the denitration catalyst in Example 1 obtained bythis sol-gel process was “V₂O_(5_)SG_300”. Denitration catalystsobtained at different molar ratios of vanadium and oxalic acid whenammonium vanadate is dissolved in an oxalic acid solution will bedescribed later.

Comparative Example 2

Ammonium vanadate was added to an oxalic acid solution and stirred for10 minutes, and titanium oxide serving as a carrier was gradually added.Then, water in the solution was evaporated on a hot stirrer and dryingwas performed in a dryer at 120° C. for one night. Subsequently, thedried powder was fired in the air at 300° C. for 4 hours. As a result,the denitration catalyst after firing that contained 0.3 wt % ofvanadium pentoxide was used as a denitration catalyst in ComparativeExample 2. The sample name of the denitration catalyst in ComparativeExample 2 was “0.3 wt % V₂O₅/TiO₂”.

Comparative Example 3

The denitration catalyst after firing that was obtained by the samemethod as in Comparative Example 2 and contained 0.9 wt % of vanadiumpentoxide was used as a denitration catalyst in Comparative Example 3.The sample name of the denitration catalyst in Comparative Example 3 was“0.9 wt % V₂O₅/TiO₂”.

Reference Example 3

The denitration catalyst after firing that was obtained by the samemethod as in Comparative Example 2 and contained 3.3 wt % of vanadiumpentoxide was used as a denitration catalyst in Reference Example 3. Thesample name of the denitration catalyst in Reference Example 3 was “3.3wt % V₂O₅/TiO₂”.

Reference Example 4

The denitration catalyst after firing that was obtained by the samemethod as in Comparative Example 2 and contained 9 wt % of vanadiumpentoxide was used as a denitration catalyst in Reference Example 4. Thesample name of the denitration catalyst in Reference Example 4 was “9 wt% V₂O₅/TiO₂”.

Reference Example 5

The denitration catalyst after firing that was obtained by the samemethod as in Comparative Example 2 and contained 20 wt % of vanadiumpentoxide was used as a denitration catalyst in Reference Example 5. Thesample name of the denitration catalyst in Reference Example 5 was “20wt % V₂O₅/TiO₂”.

Reference Example 6

The denitration catalyst after firing that was obtained by the samemethod as in Comparative Example 2 and contained 33 wt % of vanadiumpentoxide was used as a denitration catalyst in Reference Example 6. Thesample name of the denitration catalyst in Reference Example 6 was “33wt % V₂O₅/TiO₂”.

Example 2

The denitration catalyst after firing that was obtained by the samemethod as in Comparative Example 2 and contained 43 wt % of vanadiumpentoxide was used as a denitration catalyst in Example 2. The samplename of the denitration catalyst in Example 2 was “43 wt % V₂O₅/TiO₂”.

Example 3

The denitration catalyst after firing that was obtained by the samemethod as in Comparative Example 2 and contained 80 wt % of vanadiumpentoxide was used as a denitration catalyst in Example 3. The samplename of the denitration catalyst in Example 3 was “80 wt % V₂O₅/TiO₂”.

Comparative Example 4

An existing catalyst was used in Comparative Example 4. The existingcatalyst is a catalyst in which, for example, tungsten oxide (WO₃)(content: 10.72 wt %) and silica (SiO₂) (content: 6.25 wt %) aresupported on titanium oxide (TiO₂) (content: 79.67 wt %) and whichcontains about 0.5% of vanadium.

1.2 Evaluation

1.2.1 Powder X-Ray Diffraction

(Diffraction Method)

Powder X-ray diffraction analysis was performed with a Rigaku smart labusing Cu-Ka.

(Diffraction Result)

FIG. 1 illustrates powder XRD patterns of Example 1 (V₂O_(5_)SG_300),Reference Example 1 (V₂O_(5_)300), Reference Example 2 (V₂O_(5_)400),and Comparative Example 1 (V₂O_(5_)500). FIG. 2 illustrates powder XRDpatterns of Example 1 (V₂O_(5_)SG_300) and Example 2, Reference Examples3 to 6, and Comparative Examples 2 and 3 (xwt % V₂O₅/TiO₂). In thepowder XRD patterns of Example 1 (V₂O_(5_)SG_300), Reference Example 1(V₂O_(5_)300), Reference Example 2 (V₂O_(5_)400), and ComparativeExample 1 (V₂O_(5_)500), only peaks for V₂O₅ were observed regardless ofthe thermal decomposition temperature and the production method. In thepowder XRD patterns of Example 2, Reference Examples 3 to 6, andComparative Examples 2 and 3 (xwt % V₂O₅/TiO₂), peaks for V₂O₅ were notobserved at 9 wt % or less and thus V₂O₅ is believed to be highlydispersed in TiO₂. When the amount of V₂O₅ supported was increased to 20wt %, peaks for V₂O₅ were observed at 22.2° and 27.4°, and the V₂O₅ peakintensity increased as the amount of V₂O₅ supported was increased. Onthe other hand, the TiO₂ peak intensity tended to decrease.

1.2.2 Measurement of BET Specific Surface Area

(Measurement Method)

The BET specific surface area was measured with a MicrotracBELBELSORP-max. Pretreatment was performed in an Ar atmosphere at 200° C.for 2 hours, and then measurement was performed at 196° C.

(Measurement Result)

TABLE 1 BET specific surface area of vanadium pentoxide catalyst BETspecific Sample surface area/m²g⁻¹ Reference Example1 (V₂O₅_300) 16.6Reference Example2 (V₂O₅_400) 13.5 Comparative Example1 (V₂O₅_500) 4.68Example1 (V₂O₅_SG_300) 62.9 Comparative Example2 (0.3 wt % V₂O₅/TiO₂)62.8 Comparative Example3 (0.9 wt % V₂O₅/TiO₂) 59 Reference Example3(3.3 wt % V₂O₅/TiO₂) 55.4 Reference Example4 (9 wt % V₂O₅/TiO₂) 54.6Reference Example5 (20 wt % V₂O₅/TiO₂) 48.3 Reference Example6 (33 wt %V₂O₅/TiO₂) 41.2 Example2 (43 wt % V₂O₅/TiO₂) 49.4 Example3 (80 wt %V₂O₅/TiO₂) 34 Comparative Example4 (Existing catalyst) 61.8

Table 1 shows BET specific surface areas of Reference Example 1(V₂O_(5_)300), Reference Example 2 (V₂O_(5_)400), Comparative Example 1(V₂O_(5_)500), Example 1 (V₂O_(5_)SG_300), Comparative Examples 2 and 3,Reference Examples 3 to 6, and Examples 2 and 3 (xwt % V₂O₅/TiO₂catalyst), and Comparative Example 4 (existing catalyst). In thevanadium pentoxide catalysts obtained by thermally decomposing ammoniumvanadate, the BET specific surface area decreased with increasing thethermal decomposition temperature. That is, the vanadium pentoxide inReference Example 1 (V₂O_(5_)300) in which the thermal decomposition wasperformed at 300° C. had a maximum BET specific surface area of 16.6m²g⁻¹. The vanadium pentoxide obtained at 300° C. through a sol-gelprocess had a larger BET specific surface area of 62.9 m²g⁻¹. InReference Examples 3 to 6, Examples 2 and 3, and Comparative Examples 2and 3 (xwt % V₂O₅/TiO₂), as the amount of vanadium pentoxide supportedwas increased, pores in TiO₂ were filled and the BET specific surfacearea decreased.

1.2.3 Measurement of Catalytic Activity

(Measurement Method)

An NH₃-SCR reaction was performed using a fixed-bed flow reactor underthe conditions listed in Table 2 below. Among gases that had passedthrough the catalytic layer, NO, NH₃, NO₂, and N₂O were analyzed with aJasco FT-IR-4700.

TABLE 2 NH₃-SCR measurement conditions Amount of catalyst 0.375 mg Gasflow rate 250 mLmin⁻¹ (NO: 250 ppm, NH₃: 250 ppm, O₂: 4 vol %) (2000 ppmNO/Ar 31.3 mL min⁻¹) (2000 ppm NH₃/Ar 31.3 mL min⁻¹) (O₂ 14 mL min⁻¹)(Ar 177.4 mL min⁻¹) Space velocity 40,000 mLh⁻¹g_(cat) ⁻¹

Furthermore, the NO conversion ratio and the N₂ selectivity werecalculated from formulae below. Herein, NO_(in) represents a NOconcentration at an inlet of a reaction tube, NO_(out) represents a NOconcentration at an outlet of the reaction tube, N_(2out) represents aN₂ concentration at the outlet of the reaction tube, NH_(3in) representsan NH₃ concentration at the inlet of the reaction tube, and NH_(3out)represents an NH₃ concentration at the outlet of the reaction tube.

$\begin{matrix}{{{NO}\mspace{14mu}{CONVERSION}\mspace{14mu}{RATIO}} = {\frac{{NO}_{in} - {NO}_{out}}{{NO}_{in}} \times 100}} & \left\lbrack {{Formula}.\mspace{14mu} 1} \right\rbrack \\{{{N_{2}\mspace{14mu}{{SELECTIVITY}(\%)}} = {\frac{2*N_{2{out}}}{\left( {{NO}_{in} + {NH}_{3{in}}} \right) - \left( {{NO}_{out} + {NH}_{3{out}}} \right)} \times 100}}\left( {{2*N_{2{out}}} = {\left( {{NO}_{in} + {NH}_{3{in}}} \right) - \left( {{NO}_{out} + {NH}_{3{out}} + {NO}_{2{out}} + {2*N_{2}O_{out}}} \right)}} \right)} & \left\lbrack {{Formula}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

(Measurement Result)

FIG. 3 illustrates the NH₃-SCR activity of the vanadium pentoxidecatalysts. In the case of the catalysts obtained by thermallydecomposing ammonium vanadate, the NO conversion ratio increased as thethermal decomposition temperature was decreased. The highest activitywas exhibited in Reference Example 1 (V₂O_(5_)300° C.) in which thecatalyst was obtained at a thermal decomposition temperature of 300° C.At a reaction temperature of 200° C., a NO conversion ratio of 80% ormore was achieved when any of the catalysts in Reference Example 1(V₂O_(5_)300° C.), Reference Example 2 (V₂O₅ 400° C.), and Example 1(V₂O_(5_)SG_300° C.) was used. Furthermore, the NO conversion ratio washigher in any of Examples than in Comparative Example 1 and ComparativeExample 4.

The specific surface area of the vanadium pentoxide increases as thethermal decomposition temperature is decreased. Therefore, it isbelieved that the low-temperature NH₃-SCR activity that uses a bulkvanadium pentoxide catalyst is attributable to the BET specific surfacearea. Hence, as described above, the vanadium pentoxide was producedthrough a sol-gel process that uses oxalic acid in order to increase theBET specific surface area in Example 1. The BET specific surface area ofthe vanadium pentoxide produced through this process is 62.9 m²g⁻¹ asshown in Table 1, which is about four times larger than the BET specificsurface areas of the vanadium pentoxides produced through a thermaldecomposition process. The NO conversion ratio in Example 1(V₂O_(5_)SG_300° C.) was increased by 80% to 200% at 100° C. to 150° C.compared with the vanadium pentoxides produced through a thermaldecomposition process.

The N₂ selectivity was almost 100% at any temperature. FIG. 4illustrates, as examples, the N₂ selectivities in Reference Example 1(V₂O_(5_)300° C.) and Comparative Example 1 (V₂O_(5_)500° C.).

(Space Velocity Dependency)

In the case where the catalyst in Reference Example 1 (V₂O_(5_)300° C.)was used, the space velocity (for gas treatment) dependency was measuredby performing the selective catalytic reduction reaction under theconditions listed in Table 3 below. FIG. 5 illustrates the measurementresults. FIG. 5(a) illustrates the NO conversion ratio at a reactiontemperature of 120° C. FIG. 5(b) illustrates the NO conversion ratio ata reaction temperature of 100° C. The 80% NO detoxification was about 15Lh⁻¹g_(cat) ⁻¹ at 120° C. and about 11 Lh⁻¹g_(cat) ⁻¹ at 100° C. In anexperiment in which the space velocity was changed, the N₂ selectivitywas almost 100%.

TABLE 3 NH₃-SCR measurement conditions Reaction temperature 120 or 100°C. Amount of catalyst 0.375 g, 0.750 g, 1.5 g Total gas flow rate 250mLmin⁻¹ (NO: 250 ppm, NH₃: 250 ppm, O₂: 4 vol %, Ar balance) Spacevelocity 10-40 Lh⁻¹g_(cat) ⁻¹ Gas flow time 0.5 h

(Reaction in Coexistence with Water)

An experiment of the NH₃-SCR reaction was performed using the catalystin Reference Example 1 (V₂O_(5_)300° C.) under the conditions listed inTable 4 below at a reaction temperature of 150° C. at a space velocityof 20 Lh⁻¹g_(cat) ⁻¹. FIG. 6 illustrates a change in the NO conversionratio over time in the experiment. As a result of addition of 2.3% H₂O1.5 hours after the start of the reaction, the NO conversion ratiodecreased from 64% to 50%. The addition of H₂O did not change the N₂selectivity. The N₂ selectivity was 100%. As a result of stop of theaddition of water 3.5 hours after the start of the reaction, the NOconversion ratio increased to 67%.

TABLE 4 NH₃-SCR measurement conditions Reaction temperature 150° C.Amount of catalyst 0.375 g Total gas flow rate 250 mLmin⁻¹ (NO: 250 ppm,NH₃: 250 ppm, O₂: 4 vol %, Ar balance) Space velocity 20 Lh⁻¹g_(cat) ⁻¹

(Reaction in Coexistence with S)

Under the same conditions as those of the experiment of the reaction incoexistence with water, 100 ppm SO₂ was caused to flow through areaction gas. FIG. 7 illustrates the experimental results. No changeoccurred to the catalytic activity of NO. After the completion of thetemperature increase to 150° C., the SO₂ concentration did not decreasethough H₂O and O₂ were constantly present. Consequently, SO₂ did notreact. Accordingly, the denitration catalysts in Examples were found tohave S resistance.

(Relationship Between Amount of Vanadium Pentoxide Supported and NOConversion Ratio)

FIG. 8 illustrates the relationship between the amount of vanadiumpentoxide supported and the NO conversion ratio at each reactiontemperature. FIG. 8(a) illustrates the relationship between the amountof vanadium pentoxide supported and the NO conversion ratio at areaction temperature of 120° C. Similarly, FIG. 8(b) illustrates therelationship between the amount of vanadium pentoxide supported and theNO conversion ratio at a reaction temperature of 150° C., and FIG. 8(c)illustrates the relationship at a reaction temperature of 100° C. Ineach of the graphs, the catalyst in which the amount of vanadiumpentoxide supported is 100 wt % is the denitration catalystV₂O_(5_)SG_300 produced in Example 1. The points plotted using a squareindicate a NO conversion ratio of the existing catalyst in ComparativeExample 4. All the graphs showed that, on the whole, the NO conversionratio increased as the amount of vanadium pentoxide supported wasincreased. Herein, all the graphs showed that the catalyst in which theamount of vanadium pentoxide supported was 3.3 wt % had a higher NOconversion ratio than the catalyst in which the amount of vanadiumpentoxide supported was 9.0 wt %. Specifically, as illustrated in FIG.8(a), in the NH₃-SCR reaction at a reaction temperature of 120° C., theNO conversion ratio reached 80% when the amount of vanadium pentoxidesupported was increased to 80 wt %. As illustrated in FIG. 8(b), in theNH₃-SCR reaction at a reaction temperature of 150° C., the NO conversionratio considerably increased when the amount of vanadium pentoxidesupported was increased to 3.3 wt %. As illustrated in FIG. 8(c), in theselective catalytic reduction reaction at a reaction temperature of 100°C., the denitration catalyst in which the amount of vanadium pentoxidesupported was 80 wt % had a considerably increased NO conversion ratiocompared with the denitration catalysts in which the amounts of vanadiumpentoxide supported were 43 wt % or less.

(Relationship Between BET Specific Surface Area and NO Conversion Ratio)

FIG. 9(a) illustrates the relationship between the BET specific surfacearea and the NO conversion ratio of the denitration catalysts in whichvanadium pentoxide was supported on titanium oxide. In the denitrationcatalyst in which vanadium pentoxide was supported on titanium oxide, asthe amount of vanadium pentoxide supported was increased, the BETspecific surface area decreased, but the activity increased on thewhole. FIG. 9(b) illustrates the relationship between the BET specificsurface area and the NO conversion ratio of both the denitrationcatalysts in which vanadium pentoxide was supported on titanium oxideand the denitration catalysts in which vanadium pentoxide was notsupported on titanium oxide. In the catalysts in which vanadiumpentoxide was not supported on titanium oxide, the activity increasedwith increasing the BET specific surface area.

2. V₂O₅ Catalyst Produced Through Sol-Gel Process 2.1 Examples (Examples4 to 6 and Reference Examples 7 and 8)

In “Example 1” of the above-described “1.1 Examples and ComparativeExamples”, ammonium vanadate was dissolved in an oxalic acid solutionsuch that the molar ratio of vanadium and oxalic acid was 1:3, thenwater was evaporated, drying was performed, and the resulting driedpowder was fired. Thus, a denitration catalyst was produced. In thedenitration catalysts of Reference Example 7, Examples 4 to 6, andReference Example 8, the molar ratios of vanadium and oxalic acid wereset to 1:1, 1:2, 1:3, 1:4, and 1:5, respectively. Specifically, asdescribed above, ammonium vanadate was dissolved in an oxalic acidsolution (molar ratio of vanadium:oxalic acid=1:1 to 1:5). Afterammonium vanadate was completely dissolved, water in the solution wasevaporated on a hot stirrer, and drying was performed in a dryer at 120°C. for one night. Then, the dried powder was fired in the air at 300° C.for 4 hours. The sample names were given as “V₂O_(5_)SG_1:1” (ReferenceExample 7), “V₂O_(5_)SG_1:2” (Example 4), “V₂O_(5_)SG_1:3” (Example 5),“V₂O_(5_)SG_1:4” (Example 6), and “V₂O_(5__SG_)1:5” (Reference Example8). Herein, the “V₂O_(5_)SG_300” in “Example 1” of “1.1 Examples andComparative Examples” and “V₂O_(5_)SG_1:3” in Example 5 weresubstantially the same, but the sample name “V₂O_(5_)SG_1:3” in “Example5” was used for the sake of convenience of description. To increase theBET specific surface area, a surfactant may be added to the oxalic acidsolution. Examples of the surfactant include anionic surfactants such ashexadecyltrimethylammonium bromide (CTAB), sodium lauryl sulfate (SDS),and hexadecylamine; cationic surfactants; amphoteric surfactants; andnonionic surfactants.

2.2 Evaluation

2.2.1 Powder X-Ray Diffraction

(Diffraction Method)

In the same manner as in 1.2.1, powder X-ray diffraction analysis wasperformed with a Rigaku smart lab using Cu-Ka.

(Diffraction Result)

FIG. 10 illustrates powder XRD patterns of Reference Example 7, Examples4 to 6, and Reference Examples 8 (V₂O_(5_)SG). In the vanadiumpentoxides (Reference Examples 7, 7, and 10) produced using thesolutions having vanadium:oxalic acid ratios of 1:1, 1:2, and 1:5, onlypeaks for orthorhombic V₂O₅ were detected. In the vanadium pentoxides(Examples 5 and 6) produced using the solutions having vanadium:oxalicacid ratios of 1:3 and 1:4, an unidentified peak was detected at 110 inaddition to the peaks for orthorhombic V₂O₅. However, the peak has notbeen identified yet.

2.2.2 Measurement of BET Specific Surface Area

(Measurement Method)

In the same manner as in 1.2.3, the BET specific surface area wasmeasured with a MicrotracBEL BELSORP-max. Pretreatment was performed inan Ar atmosphere at 200° C. for 2 hours, and then measurement wasperformed at 196° C.

(Measurement Result)

TABLE 5 BET specific surface area of vanadium pentoxide catalyst BETspecific BET specific surface area before surface area after Catalystreaction/m²g⁻¹ reaction/m²g⁻¹ Reference (V₂O₅_SG_1:1) 29.9 n.d. Example7Example4 (V₂O₅_SG_1:2) 33.5 n.d. Example5 (V₂O₅_SG_1:3) 62.9 43.4Example6 (V₂O₅_SG_1:4) 57.0 n.d. Reference (V₂O₅_SG_1:5) n.d. n.d.Example8

Table 5 shows BET specific surface areas of Reference Example 7(V₂O_(5_)SG_1:1), Example 4 (V₂O_(5_)SG_1:2), Example 5(V₂O_(5_)SG_1:3), Example 6 (V₂O_(5_)SG_1:4), and Reference Example 8(V₂O_(5_)SG_1:5). As the ratio of the oxalic acid was increased, thespecific surface area increased at vanadium:oxalic acid ratios of 1:1 to1:3. When the ratio of the oxalic acid was further increased, thespecific surface area decreased. The specific surface area in Example 5(V₂O_(5_)SG_1:3) after the catalytic activity test described belowconsiderably decreased to 43.4 m²g⁻¹ compared with the specific surfacearea before the catalytic activity test.

2.2.3 Measurement of Catalytic Activity

(Measurement Method)

By the same measurement method as in 1.2.4, the NH₃-SCR activity of eachV₂O_(5_)SG catalyst was measured and the NO conversion ratio wascalculated.

(Measurement Result)

FIG. 11 illustrates the NH₃-SCR activity of each V₂O_(5_)SG catalyst.FIG. 11(a) illustrates the NO conversion ratio plotted against reactiontemperature in the NH₃-SCR reaction that uses each catalyst. FIG. 11(b)illustrates the relationship between the vanadium:oxalic acid ratio andthe NO conversion ratio at a reaction temperature of 120° C. In thecatalyst of Example 5 (V₂O_(5_)SG_1:3) having a vanadium:oxalic acidratio of 1:3, the highest NO conversion ratio was achieved. When theoxalic acid was further added, the NO conversion ratio decreased. The NOconversion ratio in Example 6 (V₂O_(5_)SG_1:4) was lower than that inExample 4 (V₂O_(5_)SG_1:2) despite the fact that the specific surfacearea in Example 6 was larger than that in Example 4.

(Relationship Between Specific Surface Area and NO Conversion Ratio)

FIG. 12 illustrates the relationship between the BET specific surfacearea and the NO conversion ratio in Examples 4 to 6 and ReferenceExample 7 (V₂O_(5_)SG), Reference Example 1 (V₂O_(5_)300), ReferenceExample 2 (V₂O_(5_)400), and Comparative Example 1 (V₂O_(5_)500). Thepoint plotted using a square indicates the relationship between the BETspecific surface area and the NO conversion ratio after the selectivecatalytic reduction reaction in Example 5 (V₂O_(5__SG_)1:3). Asdescribed above, it was shown that the highest NO conversion ratio wasachieved in the catalyst of Example 5 (V₂O_(5_)SG_1:3) having avanadium:oxalic acid ratio of 1:3.

2.2.4 Characterization by NH₃-TPD

(Measurement Method)

The amount of acid sites on the surface of the catalyst can be estimatedby NH₃-TPD (TPD: temperature programmed desorption). In a BELCATmanufactured by MicrotracBEL Corp., 0.1 g of each of the catalysts inReference Example 1 (V₂O_(5_)300), Reference Example 2 (V₂O_(5_)400),Comparative Example 1 (V₂O_(5_)500), Example 4 (V₂O_(5_)SG_1:2), andExample 5 (V₂O_(5_)SG_1:3) was pretreated at 300° C. for 1 hour while He(50 ml/min) was caused to flow. Then, the temperature was decreased to100° C., and 5% ammonia/He (50 ml/min) was caused to flow for 30 minutesto adsorb ammonia. The flow gas was changed to He (50 ml/min) and thisstate was kept for 30 minutes for stabilization. Then, the temperaturewas increased at 10° C./min and ammonia, which has a mass number of 16,was monitored with a mass spectrometer.

(Measurement Result)

TABLE 6 Measured amount of NH₃ desorbed by NH₃-TPD Amount of NH₃Catalyst desorbed/μmolg⁻¹ Reference Example1 (V₂O₅_300) 22.9 ReferenceExample2 (V₂O₅_400) 14.0 Comparative Example1 (V₂O₅_500) 5.21 Example4(V₂O₅_SG_1:2) 51.4 Example5 (V₂O₅_SG_1:3) 77.5

Table 6 shows the measurement results of the amount of NH₃ desorbed whenthe catalysts in Reference Example 1 (V₂O_(5_)300), Reference Example 2(V₂O_400), Comparative Example 1 (V₂O_500), Example 4 (V₂O_(5_)SG_1:2),and Example 5 (V₂O_(5_)SG_1:3) were used.

FIG. 13 is a graph obtained by plotting the amount of NH₃ desorbed as afunction of the BET specific surface area of each catalyst. The graph inFIG. 13 showed that the amount of NH₃ desorbed increased substantiallyin proportion to the BET specific surface area of V₂O₅. FIG. 14 is agraph obtained by plotting the NO conversion ratio as a function of theamount of NH₃ desorbed in each catalyst. The graph showed that the NOconversion ratio increased as the catalyst had a larger amount of NH₃desorbed, that is, a larger amount of acid sites on the surface of thecatalyst.

As described above, the denitration catalyst of the present inventionthat contains 3.3 wt % or more of vanadium oxide in terms of vanadiumpentoxide and has a specific surface area of 10 m²/g or more exhibits ahigh denitration efficiency at a low temperature of 200° C. or lower inthe selective catalytic reduction reaction that uses ammonia as areducing agent. On the other hand, oxidation of SO₂ is not found.

The invention claimed is:
 1. A method for recycling a denitrationcatalyst, comprising: a step of removing a used denitration catalystfrom a denitration device and then coating the used denitration catalystwith a catalyst component, wherein the catalyst component contains 43 wt% or more of vanadium pentoxide and has a BET specific surface area of30 m²/g or more, and the denitration catalyst after recycling is usedfor denitration at 200° C. or lower.
 2. The method for recycling adenitration catalyst according to claim 1, wherein the used denitrationcatalyst is coated with the catalyst component by impregnating the useddenitration catalyst with a chelate compound solution in which vanadiumpentoxide is dissolved and then performing firing.
 3. The method forrecycling a denitration catalyst according to claim 1, wherein the useddenitration catalyst is coated with the catalyst component by spraying achelate compound solution in which vanadium pentoxide is dissolved ontothe used denitration catalyst and then performing firing.
 4. The methodfor recycling a denitration catalyst according to claim 1, wherein inthe catalyst component, an amount of NH₃ desorbed by NH₃-TPD (TPD:temperature programmed desorption) is 10.0 μmol/g or more.